Why can't people manifest powers? I think Shields idea to put together an Avengers team stems from all the weird people. Stark forced them into action because he publicly announced himself as a superhero.

But who was Fury talking about? Cap's presumed dead. They haven't met Thor. Banner?
I think Shield knows about metahumans and other science projects. But none of them are viable candidates or they stayed under the radar.

Even in the comics, that's what happened. People with special abilities only started becoming superheroes after people like Iron Man and the X-men start popping up. Before you got people here and there and most didn't have powers. Most popped up and retired during WW2. With the sliding timescale. You got an even longer distance between WW2 heroes and the Avengers.

Basically I'm trying to say is just because we haven't been shown all these metahumans, doesn't mean they don't exist. It may just mean they aren't publicly showing themselves.........until now.